Paper No. 4.

Hydrographical Survey of the by John B. Tait, B. Sc.

Introduction. n order to arrive at a true estimate of the hydrographic conditions prevailing I throughout the year and from one year to another, in the area, it is obviously necessary in the first place to obtain, in at least general terms, a know­ ledge of the cycle of hydrographic events normally taking place in the North Sea and particularly in that region of it just outside the Firth itseli. The result of systematic hydrographic investigations has been to show that Atlantic Ocean water, which is characterised by a comparatively high salinity value of about 35.30 °/00 and a relatively high temperature, is continuously flowing in large volumes over the Wyville-Thompson Ridge into the Norwegian Sea. Off­ shoots from this Atlantic Stream enter the North Sea by way of the channels south of the and also round the north of these islands. The distribution of Atlantic water in the North Sea has been found to be subject to more or less regular seasonal variation. In normal years the inflow into the North Sea from the Atlantic Ocean becomes very powerful in late spring and early summer, reaching a maximum force between the months ol April and May. Thereafter a gradual decl ne in intensity of this high salinity water is observed to take place throughout summer and autumn months, reaching a minimum towards the close of the year, generally in the month of November. Abnormal conditions occur at intervals. For instance, during the winter of 1905 a second extensive Hooding of the northern North Sea by salt Atlantic water took place and again, throughout the entire year 1.908, the influx of Atlantic water into the North Sea was particularly scanty. Another feature of considerable importance has been observed, namely, a normal seasonal movement of less saline coastal waters which, during summer and autumn, spread out as a surface layer over the North Sea reaching consider­ able distances from land. On the approach of winter these surface waters recede towards the coasts of on the one hand and Continental shores on the — 49 — other. Evidence from investigations proves that in the layers beneath the surface, inshore and offshore movements take place, complementary to this outward- spreading and recession of the surface waters. It has also been found that during summer and autumn a large portion of the North Sea basin is normally flooded in the deeper layers by cold water of a salinity which shows it to be of Atlantic origin, but of an earlier date than the influx of the spring immediately previous. It would appear that this more or less stationary water on entering the North Sea in its state of high salinity becomes so cooled by contact with the waters of the latter that its great density causes it to sink to the deepest regions of the North Sea basin where it remains, subject only to slow mixing by convection with the less saline water above it and then probably only in its surface layers. On this assumption the deeper layers of the North Sea basin will be replenished by new water only when such water, by virtue of a greater density, seeks the lowest levels and forces the old water upwards into the region of convection mixing. With regard to the Moray Firth it appears that Atlantic water does not penetrate very far, if at all, into this region under normal circumstances. A line drawn between Duncansby Head and Kinnaird Head marks approximately the normal western boundary of high salinity Atlantic water. Only in abnormal years, such as in the autumn of 1905 does water of 35 »/„o salinity push so far westwards as Station Sc. 30 (58°00' N. 2°54' W.).

Temperature. A representation of the mean conditions of temperature prevailing through­ out the year in the Moray Firth has been obtained from records extending over twenty years. These records of temperature and salinity are for the most part to be found in the “Bulletin Trimestriel”. Quarterly isothermal charts have been drawn (Figs. 1—3) showing the mean characteristic temperature distribution for each of the four quarters of the year, at the surface, 20 metres’ and 50 metres’ depths. For their compilation, values were obtained in several cases by means of graphical interpolation having due regard to the physical disposition of the waters surrounding the point or small area in question. It must be borne in mind that these charts are comprehensive in character, but they indicate broadly the seasonal variations in temperature and the differences betwreen coastal and off­ shore temperatures, in the surface layers particularly, which differences give rise to a seasonal ebb and flowr of coastal waters, due to density variations. From January to March the surface temperature in the open Firth is higher than in the fresher and shallower coastal waters and there is, apparently, a move­ ment shorewards of warmer offshore surface water, due to the fact that by contact and close proximity to land, the coastal waters at this season of the year become 4 — 50 — rapidly cooled and consequently increase in density. As a result these waters tend to become more dense than the surface waters immediately offshore and sink below their level, thus inducing a drift of surface w-ater towards the coast and a concomitant drift offshore of the under water layers. This movement is still apparent during the second quarter of the year, but is checked before reaching the land by the beginnings of an opposing movement from the shore waters which have become influenced by the spring and early summer w-arming of the land and, as a result, are expanding and beginning to move off from the shore. The extent of this movement is more rapid from the south shore of the Moray Firth than from the coasts of Caithness and , owing, most likely, to the efflux from Cromarty Firth and Firth of fresher and warmer water. The mean conditions in respect of surface temperature from July to September are, broadly speaking, the reverse of those pertaining to the first quarter of the year. The highest temperatures occur near to the coast, especially in Cromarty Firth and along the southern shore of the Moray Firth where temperatures of 13° C. and over are recorded at the sur­ face. In the last quarter, while the open Firth surface waters cool down gradually, the water in proximity to the land soon loses its summer heat and the extended layer of surface wTater once more recedes landwards. The isotherms for depths of 20 metres and 50 metres as represented in Figures 2 and 3 show that, during the first quarter of the year, the temperature in mid-Firth rises only very gradually from the surface downwards. The rise, although very small, is sufficient to indicate that over this period of the year the sur­ face layers even in the open Firth are slightly cooler than those underneath. Nearer the coast line, however, differences in temperature between surface and 20 metre layers are appreciable (0.5°—0.75° C.). From the second quarter charts it is apparent that the surface layers are warmer than those at 20 metres which, in their turn, register temperatures about 0.25° C. higher than at 50 metres’ depth. The third quarter charts indicate very clearly a movement in the waters beneath the surface, complementary to the outw'ard spreading of the rapidly heated surface coastal waters. Those deeper wraters are, at 20 metres, almost 0.75° C. cooler than at the surface and 0.5°-—1.0° C. warmer than at 50 metres. The temperatures during the last quarter of the year are nearly constant from the surface to a depth of 50 metres at least, in the open Firth, but in the and Cromarty estuaries the surface water is distinctly cooler than that at 20 metres’ depth. In Figure 4, curves are drawn showing the mean monthly variation in sur­ face temperature at typical points in the Moray Firth. It is at once apparent that inshore waters have a greater annual range of temperature than the surface waters of the open Firth. For instance, Burghead Bay surface waters have an annual temperature range of nearly 10.5° C., those about Station Sc. 28 (57°33' N. 3°48'W.), a little farther offshore, 8.5° C. and as we go still further out the ranges fall accord­ ingly, that at Station Sc. 30 (58°00' N. 2C54' W.) being about 7.5° C. and at Station TEMPERATURE ("C.) STN. 28

6URG- e d hea y a b

STN. STN. 30 13 12 10 4 9 8 5 b 7 3

32

gur 4. re u ig F 51 MONTH — 4* — 52 —

Sc. 32 (58°08' N. 2°00' W.) just under 7.0° C. Moreover, the curves show that the times of maxima and minima vary according to distance from the land, those waters around the coast reaching their highest and lowest temperatures in advance of the waters of the open Firth. Normally, the minimum temperature for inshore surface waters occurs about the middle of February and is approximately 4° C., while a minimum of about 5.8° C. is not reached at Station Sc. 32 until the latter half of March. Similarly, with regard to maximum surface temperatures the last week of July or first week of August appears to be the time when high temperatures of nearly 14.5° C. are attained in inshore areas, w'hile at Station Sc. 32 the normal maximum temperature of about 12.7° G. is reached only towards the end of August.

Salinity. Isohalines, entered upon the same charts as the isotherms, represent the mean quarterly conditions prevailing in the Moray Firth with regard to salinity. A glance at the general disposition of the isohalines in Figs. 1—3 is sufficient to indicate that seasonal variations in salinity are much less marked than in the case of temperature. In fact the annual range of salinities at any one station is small and only in the shallow waters round the coast and near the estuaries of rivers does there appear to be any decided fluctuation. This fluctuation, irregular as it may seem to be at present, will no doubt resolve itself broadly into a seasonal change when studied in detail over a long period of years. The mean annual range in salinity of the surface waters of the Firth is from, about 33.5 °/00 to 34.95 °/oo- As already stated, the greatest seasonal variation occurs in the estuaries and in shore waters. Much lower salinities than 33.50 °/00 are sometimes met with in Cromarty and Dornoch depending upon the volume of fresh water carried down by the rivers. From a mean value of 33.75 °/00— 34.00 °/00 during the first quarter of the year the surface waters in this region and also the shallow waters of Burghead and Spey Bays show- a distinct diminution in salinity in the second and third quarters consequent upon the inflow of warmer and fresher water from the land. When autumn is reached, these waters, which, during summer, extend over the surface of the sea thus inducing an undercurrent of salter w'ater towards the shore, have had time to mix by convection with the salter under-water and, as a result, the mean salinity in this region during the last quarter of the year rises again to approximately 34.00 °/00. About mid-Firth the change in salinity of surface waters does not become really noticeable until the third quarter of the year when the position of the 34.75°/00 isohaline is considerably nearer the land than during the first six months of the year. In view of the outward spreading movement of surface shore waters during spring and summer this fact is explicable only from the point of view of convection currents. — 53 —

In the outer Firth the salinity remains very nearly constant until the third quarter when an increase of approximately 0.1 °/oo salinity is observed in the sur­ face layers. This increase continues into the last quarter of the year, reaching an average value of almost 35.00 u/oo- Although, as represented on the charts, 34.95 °/00 is the highest mean salinity value for the surface waters of the open Firth in the neighbourhood of its eastern boundary, salinities of 35.00 °/oo— 35.25 °/oo d° occur more or less frequently in this region during summer and sometimes early autumn months. In the estuaries and in mid-Firth the salinity at 20 metres’ depth is greater, more or less, than at the surface during the entire year, the difference being greater nearer the land. The waters of the outer Firth appear to be normally homogeneous with regard to salinity to a depth of 20 metres at least, throughout the year. The isohalines at a depth of 50 metres, represented in Fig. 3, show at once that the water at this depth is, over all the Firth, more salt than that at 20 metres and again the greatest differences in salinity take place between the second and third quarters and between the fourth and first quarters. In the former case it is a marked increase in salinity which is observed at the various depths and in the latter an equally rapid diminution. Mention has been made above of the presence during summer and autumn of cold dense water in the deeper regions of the northern North Sea. While the water of salinity 35.25 °/00—35.30 °/00 and temperature several degrees below that at the surface (about 7° C. generally) does not come within the limits of the Moray Firth as already defined by the line joining Duncansby Head and Kinnaird Head its presence during summer and autumn, under normal circumstances, in the neighbourhood of Station 34 (58°17' N. 1°03' W.) at depths below 50 metres has an undoubted effect upon the physical characteristics of the deeper waters in the outer Moray Firth. Station 34 is some 30 miles east of Station 32 on the same line of stations as Sc. 28—Sc. 32. A study of the disposition of both isotherms and isohalines in vertical section along this line at once reveals a gradual transition in temperature and salinity westwards from Station Sc. 34 and well into the Moray Firth area.

Currents. It has been established as a result of extensive investigations in connection with the drift of the water in the northern North Sea that the greatest influence in the Moray Firth proceeds from the water entering the Firth from the north in a south to south-westerly direction. This main drift stream divides into two portions off the southern shore of the Firth along the meridian 3U W., the lesser portion deviating westwards towards the Dornoch Firth and then north and north-eastwards along the shores of Sutherland and Caithness, thus forming an eddy in this area of the Firth where the greater part of the fishing for spawning — 54 — plaice and cod is carried on during spring months. It, is probable that minor eddies are formed in the estuaries of the Cromarty and Dornoch Firths. The bulk of the drift stream turns eastwards and moves along the southern shore of the Moray Firth towards Kinnaird Head. At certain times in some years, however, it has been observed that the drift along the southern shore of the Firth is not easterly but westerly from Kinnaird Head.

Conclusion. In the light of the above survey we are fully justified in regarding the Moray Firth, not as a large estuary like the Firths of Forth and Tay, but rather as a bay of the North Sea. As such, both isotherms and isohalines are convex to the land and it is evident from the data that in passing from the North Sea proper into the Moray Firth area there is a gradual transition in the physical characteristics of the sea. — 55 —

Figure 1. Moray Firth. Mean Quarterly Isotherms and Isohalines at the Surface.

January - .March] April - June — 56 —

Figure 2. Moray Firth. Mean Quarterly Isotherms and Isohalines at 20 metres.

[January - Marc. June

October 57 —

Figure 3. Moray Firth. Mean Quarterly Isotherms and Isohalines at 50 metres.

Apr il - JuneJanuary - March] April - JuneJanuary